The present invention relates generally to a process for impregnating corks for use in bottles or like containers, in such a way as to improve the sealing action thereof.
When storing liquids such as and more particularly wines and spirits in bottles or like containers which are closed by means of a cork, it is relatively frequently observed that the cork becomes partly or completely soaked with the liquid in the bottle. Ultimately such wetting of the cork can also result in the liquid slowly leaking from the bottle. Particularly when the liquid contained in the bottle is wine, such wetting of the cork, due to accompanying oxidation phenomena and exchange diffusion of substances between the cork and the wine, may result in undesired variations in the taste of the wine and ultimately may result in the wine having to be prematurely dumped.
The structure of cork is in substance comparable to that of a closed-pore foam material. In both cases, the cells or the pores are filled with air or gas and the cell volume is sealed off relative to adjacent cell volumes by the walls of the cell structure. The main constituents of the cell walls are wood and cellulose (about 30%), water (about 10%) and water-repellent substances (about 57.5%). The water-repellent substances are primarily in the form of saturated and unsaturated fatty acids. In addition there are also primary alcohols having from 20 to 26 carbon atoms and alkanes having from 16 to 34 carbon atoms (see for example Stazione Sperimentale Del Sughero Tempio Pausanio, ANTONIO PES, `UN ULTERIORE CONTRIBUTO ALLA CONOSCENZA DELLE CARATTERISTICHE CHIMICO FISICHE DEL SUGHERO AL FINI DEL SUO IMPIEGO IN ENOLOGIA' (translation: A FURTHER CONTRIBUTION TO KNOWLEDGE OF THE CHEMICAL-PHYSICAL CHARACTERISTICS OF CORK FOR THE PURPOSES OF USE THEREOF IN OENOLOGY), taken from No 12 of `Enotecnico` 1977, 2nd edition, ASSOCIAZIONE ENOTECNICI ITALIANI, Italian Section of the Union Internationale des Oenologues (International Union of Oenologists), Viale Murillo, 17-20149 Milan).
On the basis of investigations which are now verified (see STAZIONE SPERIMENTALE DEL SUGHERO, Settore Chimico-Technologico, TEMPIO PAUSANIO, COLLANA TECNOLOGICA, No 13, ANTIONIO PES, Criteri di scelta dei turaccioli di sughero per 1'imbottigliamento di vini pregiati, (translation: CRITERIA FOR THE SELECTION OF CORK STOPPERS FOR BOTTLING VALUABLE WINES), SASSARI TIP. GALLIZZI, 1980), in order to attain a long-term sealing action in respect of a cork in the neck of a bottle, it is necessary for about 300 g of water-repellent substance (hereinafter referred to simply as `fatty acid` as the typical main constituent) to be present per liter of volume occupied by the cork in the neck of the bottle, more particularly in the region of the side of the cork which is towards the wine. With such a fatty acid content in the cork compressed in the neck of the bottle, it is possible reliably to prevent the cork from becoming soaked by the wine even when it is stored for many years. In theory, it is always possible to achieve that value by suitable selection of the degree of compression of the cork and the contact pressure produced thereby, that is to say by suitably selecting the ratio between the diameter of the cork prior to being put into the neck of the bottle, and the diameter of the neck of the bottle. In practice however there are limits. Existing bottling plants can frequently only handle corks which are up to 25 mm in diameter. The diameters of the necks of bottles are generally in the range of from 18 to 20 mm and taper outwardly by a further 1 to 2 mm at a 40 mm depth (which is the typical length of cork). There is therefore an upper limit, defined by technical considerations, in respect of the maximum degree of cork compression that can be achieved. In addition, from the aspect of the bottle also, there is an upper limit set in respect of the maximum permissible degree of compression of the cork in the neck of a bottle as, with excessively high levels of cork compression, the internal pressure which is instantaneously built up in the operation of fitting the cork cannot decrease again and thus results in the bottle breaking. Certain shapes of bottle, in particular the box bag shape, are particularly sensitive in that respect.
Because of the upper limit, which is set by technical factors, in regard to the maximum permissible degree of cork compression in the neck of a bottle, the above-mentioned amount of 300 g of fatty acid per liter of volume occupied by the cork in the neck of the bottle, and thus a reliable long-term sealing effect, can only be achieved with corks which are of higher quality. High quality in this context denotes cork from a slowly grown cork bark with an annual ring spacing of about 2 mm, a correspondingly fine cell structure and a high weight. Lower-grade qualities of cork come from cork barks which are grown more quickly, with a correspondingly coarser annual ring spacing, coarser cell structure and lower gravity. On the other hand, irrespective of the specific weight of the cork, the fatty acid always occurs at a value of 57.5% in the cork; therefore this involves a higher degree of compression for a low-grade cork than for a high-grade cork in order to arrive at the above-indicated amount of 300 g of fatty acid per liter of volume occupied by the cork in the neck of the bottle.
Now, in order to be able to use also lower-grade qualities of cork for the long-term storage of bottle wines and the like, it is obvious that the fatty acid content should be increased by a suitable impregnation operation. It will be appreciated in that respect that such an impregnation operation would have to be carried out with substances which are repellent in respect of aqueous solutions and which in regard to their sealing effect and in regard to their compatibility with the material in the bottle, must be comparable or identical to the substances which are repellent in respect of aqueous solutions, as are found naturally in the cork. Therefore, the impregnation material must be required to be non-volatile, acceptable from the point of view of the laws relating to foodstuffs, tasteless and odorless, and it must also not react chemically with the bottled material, inasmuch such reactions would give rise to compounds which would not comply with the requirements in respect of compatibility.
Unfortunately, hitherto cork has successfully resisted all pertinent impregnation processes. It was only possible to achieve an impregnation effect on the surface of the cork or in regions of the cork which are at any event close to the surface thereof, amounting to only fractions of a millimeter. Accordingly, such a surface impregnation process could hitherto not achieve any substantial improvement in the sealing action of a cork in a bottle.